We are studying certain properties of lysogens containing defective (N minus) mutants of bacteriophage lambda. The death of lysogens after derepression of the prophage requires endoreplication initiating in the phage genome. Both autoradiography of bacterial DNA and electron microscopy of F's containing derepressed lambda will be used in an attempt to demonstrate lambda endoreplication directly. Heating lysogens containing certain lambda N minus cIts prophages does not result in the death of the lysogens, although the equivalent N plus prophages replicate and kill the host. Evidently certain ts repressors retain some activity at 43 degrees C. The level of repressor activity in lysogens will be monitored using a lambda trp/lac superinfecting phage in which lac expression is controlled by the repressor of the prophage. The stability of polylysogens of lambda is also under study.